There is a conventional activation device for a vehicular protection system disclosed in Patent Document 1. This activation device includes an acceleration sensor, a micro-computer, a squib (ignition element), two electronic switching elements, and a driving circuit that drives the foregoing components.                Patent Document 1: JP-2001-239916 (US-2001/0006309 A1, U.S. Pat. No. 6,465,907 B2)        
The acceleration sensor detects impact from vehicle collision; then, the micro-computer turns on the two electronic switching elements via the driving circuit based on the detection result by the acceleration sensor. Thus, ignition current is conducted to the squib from a direct-current power source to thereby expand an airbag device. An occupant can be thereby securely protected from the impact due to the collision.
Here, the squib and the switching elements are separately disposed and connected with each other via a wire harness. Therefore, they are subject to interference due to inductive noises from various electronic devices mounted in the vehicle, so the switching elements are sometimes damaged because of the inductive noises.
To solve this problem, a noise protection circuit 70 shown in FIG. 4 is provided for protecting the switching elements from the noises. This circuit 70 includes three zener diodes 70a, 70b, 70c. The first zener diode 70a is connected to the squib 60 in parallel: the second and third diodes 70b, 70c are individually connected between either end of the squib 60 and the vehicle body. This structure suppresses voltage variations, due to the inductive noises, between both ends of the squib 60 and between either end of the squib 60 and the vehicle body. The damage of the switching elements 50a, 50b can be thereby prevented. However, this circuit includes three zener diodes that are large and relatively costly, so it is difficult to make this circuit cheap and compact.